Note: the following constitutes my own informal understanding and interpretation of Gnosis. This is a monistic, panentheistic interpretation quite at variance with (but not contradictory to) the more restricted academically accepted and dualistic version of the term. I see it as a subset (or one of numerous possible subsets) of the more universal and mystical understanding of Gnosis presented here.
Gnosis is the Light Within. It is the voice of the Higher Self. It is the only true Guru, the only true Teacher. It deals with matters that are Absolute. There are many who follow an external guru or teaching, because they do not have the Light Awakened within them, and require to be told by another what is Absolute. For me, there are many enlightened teachers (and even more that are not so enlightened), whom I respect. But respecting them does not mean slavish imitation.
Gnosis articulated is what is known as the Perennial Philosophy. Note, I don't necessarily agree with every perspective or author mentioned or cited, but with the general sentiment.
One way in which I differ from the standard religious-based Perennialist postion is in non-harmfulness to all sentient beings, not just humans. This was taught by Buddha and Mahivira in India, by Pythagoras in the classical west, and St Francis of Assisi in the Christian world. For me, this non-anthropocentric perspecticve also implies not merely a vegetarian but a vegan lifestyle. However, while that is what my gnosis compells me to do, I am not claiming it as absolute that should be forced. If one feels that way, one will act in such a way, to walk lightly on the Earth, with the right ecological attitude, and not harm other beings; to have a right attitude to humans, animals, plants, even inanimate nature (interestingly, the Baal Shem Tov spoke of sparks in even inanimate matter that need to be "raised up"). One should be guided by the inner light, the True Light, and then one will act correctly, regardless.
For me, ahimsa, non-harmfulness, does not mean one should stand by idly and let innocents or loved ones be harmed because you don't want to harm the aggressor. To me, that is just stupid. Strength is a part of the Divine Plan as much as Empathy and Compassion. In this the Kabbalists and Hermeticists are absolutely correct, I feel. Interestingly, Sri Aurobindo also was critical of the non-violence advocated by Gandi as a universal solution, and I see where he is coming from. Gandhi's peaceful protest worked with the British, who are a democratic nation with a sense of justice and human rights. Martin Luther King also was able to bring about civil rights for afroamericans because, again, America is a nation based on these principles. But peaceful protest does not work against, say, the Communist Chinese, as they are a totalitarian regime who care neither for their own dissidents nor for other people (e.g. the Tibetans) they wish to repress.
One very big problem many mystical teachings have, which to me is an abberrationa nd not part of true spirituality or genuine (as I experience it) gnosis, is their guilt trips and hang-ups over the the physical body and associated lower emotional tendencies. We see this in exoteric religions with their obsession with "sin" and guilt (Catholic Church private schools that in the past would teach girls when they shower not to look at their bodies being only one such example), many spiritual teachings (many yogis and mystics have hang-ups with asceticism, hatred of the body etc), blanket rejection of the ego, using the same attitude of "that is bad" that those exoteric religions do to the body (this is typical of a vast number of pop Gurus with little true insight), fear of the mind and of reason (leave your mind at the door (Osho Rajneesh), elements in the New Age movement, a young lady in the rebirthing movement I knew some time back (no, she wasnt my girlfriend)) etc etc.
The true approach is not denial, but transformation and transmutation. e.g. the Vajrayana teaching of changing the five poisons into five wisdoms. Also Taoist internal Alchemy, working with the flow of ch'i. And Sri Aurobindo speaks of (his) yoga as an "integral" transformation of all faculties of the being.
Yet Gnosis, perhaps, and despite its core of unity, is a profoundly individual thing, and what Gnosis is for me may not be what Gnosis is for you. But this is how I understand, interpret, and experience it.
I wrote the following definition (slightly edited here) in reply to comments to a blog post
By Gnosis I mean an inner certainty about spiritual realities that has been very real to me since i was only 20 or 21. It pertains to the inner mind (to use Sri Aurobindo's terminology). I have never been a “seeker”, because a seeker doesn't know. Rather I have - ever since the "scales fell from my eyes" (wonderful saying, from Acts 9:18) when i was about 20, known, not in a rational sense., But in a sense that when i read spiritual knowledge it was like a remembering. A bit like in the Platonic sense that all true knowledge and education is remembering what we knew before. When I read Tibetan Buddhism, Jung, Sri Aurobindo, Henry Corbin, whoever, ah - of course, how obvious! Why didn't I see that before. That's what I mean when I speak about Gnosis. I have found on forums like Open Integral it is impossible to convey this concept.
But my gnosis is just one form of gnosis, one facet of gnosis. And authentic mystical experience is also gnosis (obviously more profound than mine). An example of this is enlightenment, for example the state of being of Ramana Maharshi. The ascent to the Supermind in Aurobindonian Integral Yoga is an even more profound gnosis again. Sri Aurobindo even uses the term gnosis. So I wouldn't say that gnosis is only this but not that. There are many forms and types and grades of gnosis.
Two things can be said about gnosis
Of course it is easy for the ego to hijack gnosis, then you have the “intermediate zone, the mixture of truth and falsehood that characterises most gurus.
See also my suggestion for an Integral Gnostic Community - posted on Zaadz ; the same posted on Integral Transformation and comments by others